The embodiments relate to Chronic Obstructive Pulmonary Disease (COPD) diagnosis and, more particularly, to an image diagnostic apparatus using chest CT images. COPD is a disease involving chronic bronchitis and emphysema. In the United States, in particular, COPD is the fourth leading cause of death of persons aged from 65 to 84 years, following the three major diseases (cancer, heart disease, and stoke), and is expected to become the third leading cause of death in place of stoke by the year 2020. Chronic bronchitis indicates persistent or recurrent excess bronchial mucus secretion in the bronchi because of a chronic inflammation of the trachea or bronchi. With regard to a bronchial structure, the fibrous tissue narrows the lumen. In addition, emphysema is a state in which the peripheral alveoli/alveolar duct are destroyed and fused to produce bulla to result in the loss of elasticity. Both the diseases are disorders in which the peripheral small bronchi inflated by the tension of the alveoli are obstructed to result in a deterioration in expiratory ability.
COPD clinical diagnosis is performed by spirometry using the expiratory flow rates measured by a spirometer as a respiratory function examination device. In this case, COPD is diagnosed if the ratio (1 second ratio: FEV1%) of the expiratory volume 1 sec after the administration of a bronchodilator to the forced vital capacity (FVC) is less than 70%. In addition, the severity of COPD is diagnosed based on the ratio (%1 second ratio: % FEV1) of the FEV1 to the standard FEV1. Severities are expressed by Global Initiative for Chronic Obstructive Lung Disease (GOLD) values (GOLD 0 (risk group) to GOLD 4 (most severe)) defined by an international project called GOLD. COPD diagnosis by this spirometry is limited to the diagnosis of an overall lung field function. For this reason, in order to make a treatment strategy, treatment effect determination, and follow-up, there are demands for local or localized COPD diagnosis for each pixel of a medical image or for each lung area such as a lung lobe or pulmonary segment.
Under the circumstances, there has already been realized a technique of expressing the degree of local COPD by presenting a Low Attenuation Area (LAA) representing a change in the above bronchial structure and the presence of bulla caused by COPD using a chest CT image at the time of expiration. However, since this technique is designed to perform determination based on a given threshold (e.g., −950 HU) at a given CT value, the technique cannot be said to be an accurate diagnosis technique because CT values are not always common to all CT apparatuses. In order to solve this problem, as observation methods, there have been proposed a method of presenting a movement amount for each voxel obtained from the registration information obtained by performing registration between an expiration image and an inspiration image of chest CT images and a method of performing determination based on CT values obtained at the time of expiration and at the time of inspiration for each voxel upon registration (non-patent literature 1). The latter method is designed to set −950 HU for inspiration images and −856 HU for expiration images in a two-dimensional coordinate space having, as two axes, CT values (x-axis) at the time of expiration and CT values (y-axis) at the time of inspiration, and to perform determination on a pixel basis by setting normal, COPD (substantially equivalent to emphysema), and functional Small Airways Disease (fSAD) as an intermediate disease state between them with respect to the four formed quadrants.
It is however difficult to accurately determine the degree of COPD from only the motions of voxels or CT values using expiration and inspiration images of the chest CT images described above.